1. Field of the Invention
The present invention relates generally to sputter deposition on substrate surfaces. More specifically, the present invention relates to methods and apparatus for measuring the profile of a sputtering target.
2. State of the Art
A thin film of metallic material may be deposited on a substrate using a sputter deposition process wherein a metallic target is attacked with ions causing atoms or small particles of the target to be ejected from the target and deposited on the substrate surface. FIG. 1 illustrates a cross-sectional schematic of a typical sputtering apparatus 10 comprising a vacuum chamber 12 having a gas inlet 14 and a gas outlet 16. The sputtering apparatus 10 further comprises a substrate support pedestal 24 and a metallic target 22 attached to a sputtering cathode assembly 18, each located within the vacuum chamber 12. The pedestal 24 may be configured to secure a substrate 26 thereto with a biasable electrostatic chuck, a vacuum chuck, a clamping structure, or a combination of methods. The substrate 26 may be transported to and from the pedestal 24 manually or with a robotic arm or blade (not shown).
During the sputtering process, the vacuum chamber 12 is filled with an inert gas, such as argon, through the gas inlet 14 and then reduced to a near vacuum through the gas outlet 16. The target 22 is negatively charged to cause electrons to be emitted from an exposed surface 23 of the target 22 and move toward an anode (not shown). A portion of the moving electrons strike atoms of the inert gas, causing the atoms to become positively ionized and move towards the negatively charged target 22. The electrons, inert gas atoms, and ions form a plasma which is typically intensified and confined over the target surface 23 by a magnetic field generated by a magnet assembly 20 located proximate the target 22. The magnet assembly 20 may comprise one or more permanent magnets or electromagnets located behind and/or to the side of the target 22. A portion of the ions discharging from the plasma strike the target surface 23 at a high velocity, causing atoms or small particles of the target 22 material to be ejected from the target surface 23. The ejected atoms or small particles then travel through the vacuum chamber 12 until they strike a surface, such as the surface of the substrate 26, forming a thin metallic film thereon.
The magnetic field formed over the target surface 23 by the magnet assembly 20 confines the electrons emitted from the target 22 to an area near the target surface 23. This greatly increases the electron density and the likelihood of collisions between the electrons and the atoms of the inert gas in the space near the target surface 23. Therefore, there is a higher rate of ion production in plasma regions near the target surface 23 where the magnetic field intensity is stronger. Varying rates of ion production in different plasma regions causes the target surface 23 to erode unevenly. Typically, the configuration of the magnet assembly 20 produces a radial variation of thick and thin areas, or grooves, within a diameter of the target surface 23. FIG. 2 illustrates a cross-sectional perspective view of a typical erosion profile of a cylindrical metallic target 22, such as the metallic target 22 shown in FIG. 1, which has been used in a sputtering process. FIG. 2 illustrates a target surface 23 before erosion has occurred as well as a target surface 32 that has eroded unevenly across the length of a diameter bounded by an outside edge 25 of the target 22. Due to the geometry of a magnetic field surrounding the target 22, the target surface 32 has eroded nearly symmetrically about a center line 30 dividing the length of the diameter.
Referring now to FIGS. 1 and 2, the target 22 may comprise a rare metal, such as gold, platinum, palladium or silver, or may comprise, for example, aluminum, titanium, tungsten or any other target material conventionally employed in the semiconductor industry. Therefore, it is advantageous to consume as much of the target 22 material during sputter deposition processes as possible before replacing an eroded target 22. Further, replacing an eroded target 22 before the end of its useful life may be a difficult and time-consuming task. However, it is important to replace the target 22 before a groove xe2x80x9cpunches throughxe2x80x9d the target 22 material and exposes portions of the cathode assembly 18 to erosion, causing damage to the cathode assembly 18 and contaminating the sputtering apparatus 10. For example, the target 22 material in the area of groove 28 shown in FIG. 2 may erode before the remainder of the target 22 material and expose the cathode assembly 18 to ionic bombardment from the surrounding plasma.
The useful life of a metallic sputtering target 22 is typically estimated by determining the cumulative deposition time for the target 22. A deposition time is chosen to guarantee that the target 22 material will never be completely removed at any given location and may take into account the thickness of the target 22, the material used for the target 22, and the effect of intensifying and confining the plasma over the target surface 32 by a magnetic field generated by the magnet assembly 20 in a predetermined configuration. However, if the characteristics of the plasma distribution change due to, for example, reconfiguring the magnet assembly 20 to produce a magnetic field with a different geometry, the erosion of the target surface 32 may be changed and could result in localized enhanced metal removal and the possible punching through to the cathode assembly 18 before the expiration of the estimated deposition time.
Directly measuring the target surface 23 is difficult and time consuming. Opening the vacuum chamber 12 to inspect the target surface 23 requires several hours of idle time while the vacuum chamber 12 is baked out under post-vacuum inspection. Accurate measurement of the target surface 23 while the sputtering apparatus 10 is under vacuum is difficult because the gap distance d between the target 22 and the pedestal 24 may be as small as 25 millimeters. Typical measurement devices are too large to be inserted into the gap d between the target 22 and the pedestal 24 to profile the target surface 23 while the vacuum chamber 12 is under vacuum. Further, measurement devices placed near the target 22 during a sputtering process may be damaged by exposure to metal deposition.
In view of the shortcomings in the art, it would be advantageous to prevent premature replacement and overconsumption of the target 22 by providing a technique and device to measure the target surface 23 while the vacuum chamber 12 is under vacuum.
The present invention relates to methods and apparatus for measuring the erosion of a metallic sputtering target.
An apparatus according to one embodiment of the present invention comprises a sensor configured to emit an energy beam toward a target surface and to detect a reflection of the energy beam from the target surface. The sensor may be coupled to a thin profile arm configured to move or transport the sensor over the target surface between the target and a substrate support pedestal to a plurality of measurement locations. The arm may be configured to attach to a robotic device. The sensor and the arm are configured, positioned and sized to be inserted into a narrow gap existing between the target surface and the pedestal. The arm may also be configured to remove the sensor from the gap and to shield the sensor during a sputtering process.
In another embodiment of the present invention, the sensor comprises a source element configured to emit a collimated light beam and a plurality of detectors arranged in a linear array. The source element may be positioned so as to emit the collimated light beam at an acute angle with respect to the linear array of detectors. The detectors are positioned relative to the source element so that one of the detectors in the array will be illuminated by a reflection of the collimated light beam. The distance from the sensor to the target surface or the percentage of target erosion may be calculated by determining which detector in the array is illuminated by the reflection of the collimated light beam.
In a further embodiment of the present invention, the sensor comprises a transmitter optically coupled to a source collimator configured to collimate a light beam as it exits an optical fiber. The sensor may further comprise a receiver optically coupled to a plurality of collection collimators, each of the plurality of collection collimators being configured to collect the reflection of the light beam incident thereon into a corresponding optical fiber.
The present invention also encompasses a sputter deposition system incorporating the sensors of the present invention and a method of profiling a sputtering surface target. The method comprises emitting an energy beam, illuminating a first location on the target surface, detecting a reflection of the energy beam from the at least one location, and analyzing the detected reflection of the energy beam to determine a distance from the point of emission to the first location.
Other features and advantages of the present invention will become apparent to those of ordinary skill in the art through consideration of the ensuing description, the accompanying drawings, and the appended claims.